The use of relatively lightweight, anodized aluminum and aluminum alloys in the aerospace industry has enabled the production of lighter and less expensive to manufacture aerospace vehicles. Anodized aluminum and aluminum alloys are formed by the coating of a protective, passive layer of aluminum oxide on a surface layer thereof, typically by an anodic process in a suitable electrolyte such as chromic or sulfuric acid. However, one significant and quite common problem that has been encountered by the aerospace industry with respect to anodized aluminum and aluminum alloys is a type of corrosion generally referred to as pit corrosion.
Pit corrosion is generally defined as a localized form of corrosion, i.e., the bulk of the affected surface remains substantially unattacked. Typically, pit corrosion occurs when the affected surface is exposed to a corrosive environment, such as relatively high moisture levels, especially those containing dissolved chemical species, such as halides.
Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films (e.g., anodized surfaces). Localized pitting attack is found where these passive films have broken down. Within the pits, an extremely corrosive micro-environment tends to be established, which may bear little resemblance to the bulk corrosive environment. The pH within the pits tends to be lowered significantly, together with an increase in chloride ion concentration, as a result of the electrochemical pitting mechanism reactions in such systems. Detection and monitoring of pitting corrosion is extremely difficult. Eventually, pitting can lead to mechanical failure of the affected component, which can have catastrophic effects, especially in an aerospace vehicle setting.
In order to protect the anodized aluminum alloys from corrosion, especially pit corrosion, it was common practice in the aerospace industry either paint the surface or to seal the surface thereof by applying oxo-anions of hexavalent chromium, also commonly referred to as chromium(VI). Additionally, sparingly soluble chromium(VI) compounds have also been dispersed in a resin film in order to provide a corrosion resistant paint primer for aluminum and aluminum alloy components. Both of these methods have been widely used in the manufacture of aerospace vehicles and components thereof.
Other approaches to overcoming this problem have included high temperature water sealing and nickel acetate sealing of anodized aluminum, although the results have not been entirely satisfactory.
It is generally believed that the role played by chromium(VI) in corrosion protection is that it renders protective coatings active by releasing corrosion inhibiting chemical species upon exposure to an aqueous environment. These species then migrate to defects (e.g., pits) in the protective coatings where they inhibit further corrosion. It is also generally believed that these chromium(VI) species lower the zeta potential upon adsorption on metal oxides, such as anodized aluminum and alloys thereof. This adsorption is considered to discourage subsequent adsorption of corrosive halide anions, such as chloride anions.
Unfortunately, chromium(VI) is quite hazardous and is currently believed to be a potential human carcinogen. As such, it's continued long-term use in the aerospace industry as a corrosion inhibiting agent is in extreme doubt.
Therefore, there exists a need for an alternative corrosion inhibitor for aerospace components comprised of either painted or anodized aluminum and aluminum alloys, wherein the corrosion inhibitor is inexpensively and easily manufactured, is non-hazardous to handle, is easily applied, and is effective against corrosion, especially pit corrosion.
Furthermore, this alternative corrosion inhibitor should: (1) adsorb onto aluminum oxide coatings so as to lower the zeta potential of the oxide; (2) remain immobilized in the protective coating (i.e., paint or oxide) until needed to inhibit corrosion; (3) release corrosion inhibiting species upon exposure to a corrosive aqueous environment; and (4) reinforce the growth of a protective aluminum oxide film, or inhibit cathodic reduction of oxygen on cathodically active sites such as secondary phase compounds.